1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, a developer for developing an electrostatic image and an image forming method, and more particularly, it relates to a toner for developing an electrostatic image and a method for producing the same, a developer for developing an electrostatic image and an image forming method, which are used in an apparatus utilizing electrophotographic process used in a duplicator, a printer and facsimile machine, and particularly in a color duplicator.
2. Description of the Related Art
In recent years, the electrophotographic process is not only utilized in duplicators, but also widely applied to network printers for office use, printers for personal computers, and printers for on-demand publishing, owing to progress in equipments and enhancement of communication networks in the current information society. The printers are strongly demanded to have high image quality, high operation speed, high reliability, small size, light weight and energy saving capabilities, in both monochrome and color types.
In the electrophotographic process, an image is ordinarily formed in the following plural steps. A latent image is electrically formed on a photoreceptor utilizing a photoconductive substance by various means. The latent image is developed with a toner, and a toner image formed on the photoreceptors transferred to a transfer material, such as paper, through or not through an intermediate transfer material. Thereafter, the transferred image is fixed to the transfer material.
In general, as a contact fixing method, which has been ordinarily used as a method of fixing a toner, such a method using heat and pressure upon fixing has been utilized (hereinafter, referred to as a heat pressure method). In the heat pressure method, the surface of the fixing member and the toner image on the transfer material are in contact with each other under pressure, whereby fixation can be rapidly carried out with good thermal efficiency, and the method is particularly effective for a high-speed electrophotographic image forming apparatus.
According to the increasing demand for saving energy in recent years, toners have been investigated for low temperature fixing property to decrease electric power consumption upon fixing the toner, and among various investigations, some types of toners containing a crystalline resin as a binder resin have been reported. For example, JP-A-2002-082485 and JP-A-2000-352839 propose a toner containing a crystalline polyester resin. However, a toner using a crystalline resin as a binder resin has such a problem that the charge amount becomes low to provide insufficient developing capability.
JP-A-2001-42568 proposes a toner having such a structure that a crystalline resin is enclosed in a radical polymer, and the toner is excellent in charging property owing to the absence of a crystalline resin on the outermost surface of the toner. However, the toner has such a problem that the crystalline resin has a too low melt viscosity to bring about frequent occurrence of hot offset upon fixing.
It is considered as an effective measure that a crystalline resin is enclosed inside the toner to exert low temperature fixing property, and an electrically chargeable material is exposed on the outermost surface of the toner to exert charging property. However, it is the current situation that such a toner has not yet been obtained that the viscosity of the crystalline resin is adjusted into a suitable range, a surface satisfying developing property and transferring property is provided, and the total capability including developing property, transferring property and fixing property is satisfied.
Under the circumstances, for example, the following types of toners have been proposed. Such a toner is proposed, for example, in Japanese Patent No. 2,750,853 that contains mother particles having an effluent starting temperature of 110° C. or less and small particles coated and embedded on the surface of the mother particles. Such a toner is proposed, for example, in JP-A-5-181301 that contains a core material of a styrene-acrylate resin having a molecular weight of from 3,000 to 30,000 and a glass transition point of from 50 to 70° C. coated with a shell material of a styrene resin having a higher molecular weight and a higher glass transition point. Such a toner is proposed, for example, in JP-A-6-342224 that contains mother particles having resin fine particles for surface modification fixed thereto through mechanical impact. Such a toner is proposed, for example, in JP-A-8-254853 that contains a core material of a saturated fatty acid or a saturated alcohol having a melting point of from 40 to 100° C., which suspended in water and then encapsulated with resin fine particles. Such a toner is proposed, for example, in JP-A-9-258480 that contains low viscosity resin particles having accumulated on the surface thereof a thermally stable layer and a thermoplastic resin layer having a glass transition point of 65° C. or more. Such a toner is proposed, for example, in JP-A-2001-175025 that contains a toner containing a resin having a glass transition point of from 25 to 55° C. having attached on the surface thereof resin fine particles having a glass transition point of from 60 to 110° C. Such a toner is proposed, for example, in JP-A-8-220808 that uses a linear polyester resin having a softening point of from 90 to 120° C. and carnauba wax. Such a polymerized toner that has wax encompassed therein is proposed, for example, in JP-A-5-61242. Such a toner is proposed, for example, in JP-A-2004-198658 that contains a toner obtained by expanding and crosslinking an isocyanate group-containing prepolymer with an amine compound, on the surface of which fine particles are fixed.
However, these toners cannot be such a toner that is excellent in anti-offset property, storage stability and anti-filming property with satisfying the low temperature fixing property, which is being demanded in recent years. A capsule toner having a core-shell structure with a continuous shell layer is insufficient in low temperature fixing property, and that with a particulate shell layer as in JP-A-2002-082485 is low in viscoelasticity upon melting the toner and is insufficient in anti-offset property since it does not contain a releasing agent.
Furthermore, JP-A-2000-187358 proposes a toner having a filler contained therein for improvement in fixing property, but it is insufficient in low temperature fixing property due to a high softening temperature of the binder resin.